The present invention relates to a remote control system for data communication utilizing modems (modulator-demondulator) with multiplexing function, and more particularly to a remote control system having a function to effect port designation of a responding modem remote from an initiating modem by using a secondary channel for performing error test of communication channel.
When employing a modem having multiplexing function as a remote modem in a remote control system for data communication utilizing modems, it is required in many applications to designate one port among a plurality of ports for performing such a remote control.
In accordance with this system, when designating a port of a remote modem as a responding modem from a local modem as an initiating modem, an operation is effected to notice a multiplexed port number of the remote modem from the local modem to the remote modem via a secondary channel which is not open to users. However, the drawback with the above-mentioned conventional system is as follows. In case where an error test is conducted by looping back port designation signal at the designated port of the remote modem to feed it back to the local modem, it is required that the port number noticed or reported via secondary channel be equal to the number of the port at which the port designation is looped back. For this reason, the local modem requires information as to correlative relationship between port numbers of the local modem and those of the remote modem. As a result, an erroneous designation of port numbers will cause serious communication faults such that communication function of the channel erroneously designated is down.